Why is there different shading on the same material
For the flat faces:


Select the flat faces in edit mode and set shading to smooth
Check your auto-smooth settings (under Mesh properties -> Normals)


For the other face, It looks like you have an N-gon (a face with more than 4 edges). N-gons tend to triangulate poorly and so can lead to shading artifacts, particularly when they are not convex like the one you have. You need to break this up into simpler faces.